Even in the closely knit energy industry they are virtually unknown. On the streets of Geneva, London and Houston they go unrecognized. Yet a handful of executives were oil-industry standouts in 2015. They thrived because of — not despite — plunging crude prices.From Total SA to Trafigura Group Pte, trading emerged as the industry’s cash cow.Take Vitol Group BV, the world’s largest independent energy-trading operation. The 50-year-old company reported net profit of $1.6 billion last year — the fourth highest ever, buoyed in part by the strategies employed by the teams headed by Mark Couling, chief oil trader.“The oil trading industry as a whole enjoyed the best year since 2008-09,” said Damian Stewart, managing director at Human Capital, a commodities-focused headhunter.